Add custom field support to GELF appender

GELF supports additional fields (https://github.com/Graylog2/graylog2-docs/wiki/GELF), I added this support in 2 ways:

1. If user want to add custom fields for every message, add the key-value pairs to config.customFields
````js
log4js.configure({
    appenders: [
        {
        type: "gelf",
        host: "LOG-SERVER",
        hostname: "nodejs-app",
        facility: "nodejs-facility",
        customFields: 
            {
                _http: 404,
                _application: "My Application",
            }   
        },
        {type: 'console'},
    ]
});
````


2. If users want to add/overwrite custom fields for some message, add it as the first object to the logger call:
````js
logger({ GELF: true, _http:409, _application:"His Application" }, "This is an known issue")
````